Pass a curl_off_t as parameter. It contains the offset in number of bytes that
you want the transfer to start from. Set this option to 0 to make the transfer
start from the beginning (effectively disabling resume). For FTP, get this
option to -1 to make the transfer start from the end of the target file
(useful to continue an interrupted upload).
When doing uploads with FTP, the resume position is where in the local/source
file libcurl should try to resume the upload from and it will then append the
source file to the remote target file.